Flat valley fresnel lens

ABSTRACT

A Fresnel lens. The Fresnel lens includes an input side configured to receive the image and an output side, opposite the input side, configured to display the image. The input side includes a first facet angled to receive input light constituting at least a portion of the image. The input side also includes a second facet at least partially facing the first facet and a third facet linking the first facet to the second facet, wherein the third facet and the output side are substantially parallel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No.10/753,985 filed Jan. 6, 2004, which is aContinuation-in-Part of U.S. patent application Ser. No. 10/693,615filed Oct. 23, 2003, which is a Continuation-in-Part of U.S. patentapplication Ser. No. 10/222,083, now U.S. Pat. No. 6,896,375, filed Aug.16, 2002. The contents of the above are incorporated by reference intheir entirety for all purposes.

BACKGROUND

In order to provide a television with a screen size greater thanapproximately 40 inches a display device other than a direct viewcathode ray tube (CRT) is typically used. As the screen size of a CRTincreases, so too does the depth. It is generally accepted that forscreen sizes greater than 40 inches direct view CRTs are no longerpractical. Three alternatives exist for large screen (>40 inch screensize) displays: projection displays, plasma displays, and Liquid CrystalDisplays (LCDs).

Current plasma and LCD displays are much more expensive than projectiondisplays. Plasma and LCD displays are generally thin enough to mount ona wall, but can be heavy enough that mounting can be difficult. Forexample, current 42-inch plasma displays can weigh 80 pounds or more and60-inch plasma displays can weigh 150 pounds or more. One advantage ofplasma and LCD displays over current projection displays is that theyare typically much thinner than current projection displays having thesame screen size.

Projection displays, specifically rear projection displays, aretypically more cost-effective then plasma displays. Projection displaysmay also consume too much space in a room to provide a practicalsolution for large screen needs. For example, typical 60-inch rearprojection displays are 24 inches thick and can weigh 200 to 300 pounds.

FIG. 1 illustrates a prior art rear projection display device. Ingeneral, display device 100 includes optical engine 140, projection lens130, back plate mirror 120 and screen 110. Optical engine 140 generatesan image to be projected on screen 110. Projection lens 130 projects theimage from optical engine 140 on to back plate mirror 120, whichreflects the image to screen 110. The size of display device 100 isproportional to the size of the image to be displayed on screen 110.Thus, for large screen sizes (e.g., >60 inches), the overall size ofdisplay device 100 can be very large.

Fresnel lenses may be used to direct a projected image toward a viewer.Conventional rear projection display devices are thick because ofsurface reflections from the Fresnel surface. As the angle of incidenceincreases (on the flat side of the Fresnel) the amount of light that isreflected from the air-plastic interface also increases, reducing imageuniformity. A person of ordinary skill in the art is familiar withcalculating Fresnel surface reflections.

FIG. 2 illustrates a conventional rear projection display device 200that is implemented with a Fresnel lens. Conventional rear projectiondisplay device 200 includes: optical engine 210, projection lens 220,Fresnel lens 230, and diffusion screen 240. The light impinging on thetransmission surface of Fresnel lens 230 may be roughly symmetric withrespect to optical axis 250. The use of such Fresnel lenses may generatelight artifacts, such as stray light. These light artifacts may affectthe quality of a displayed image.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by way of example, and not by way oflimitation, in the figures of the accompanying drawings in which likereference numerals refer to similar elements.

FIG. 1 illustrates a prior art rear projection display device.

FIG. 2 illustrates a conventional rear projection display device 200that is implemented with a Fresnel lens.

FIG. 3 illustrates one embodiment of an ultra-thin rear projectiondisplay device with planar mirrors parallel to a screen.

FIG. 4 illustrates a Fresnel lens with outlines of sections to be usedfor rear projection screens.

FIG. 5 illustrates a cross-sectional profile view of a Fresnel lenshaving a groove angle of 39°.

FIG. 6 a illustrates a front view of a Fresnel lens having two zoneseach having a different groove angle.

FIG. 6 b illustrates a cross-sectional profile view of a two-zoneFresnel lens having a first zone with a groove angle of 35° and a secondzone having a groove angle of 41°.

FIG. 7 illustrates an input ray having a 60° input angle with a Fresnellens having a face angle of 10°.

FIG. 8 illustrates a profile cross-section view of a Fresnel lens designhaving two zones with different groove angles and transition regions forthe zones.

FIG. 9 illustrates a profile cross-section view of a selected portion ofa Fresnel lens design.

FIG. 10 illustrates one embodiment of a Fresnel lens having two zones onopposite sides of the lens with a transition region for the two zones.

FIG. 11 illustrates one embodiment of a rear projection display devicehaving a wide-angle lens.

FIG. 12 illustrates rear projection display device 1200 and thepotential problem of stray light.

FIG. 13 illustrates rear projection display device 1300, with elementsto reduce stray images.

FIG. 14 illustrates an exemplary bump 1410 scattering light rather thancoherently reflecting light.

FIG. 15 illustrates an exemplary diffusion layer 1510 diffusing lightthat might otherwise form a stray image.

FIG. 16 illustrates the relationship between face angle (γ) and outputray angle (β), according to an embodiment of the invention.

FIG. 17 provides an exemplary illustration of face angle (γ) and outputray angle (β) varying as a function of radial distance from the centerof a Fresnel lens, according to an embodiment of the invention.

FIG. 18 is a front view of screen 1800.

FIG. 19 illustrates rear projection display device 1900.

FIG. 20 illustrates stray light production from a Fresnel lens.

FIG. 21 further illustrates stray light production from a Fresnel lens.

FIG. 22 illustrates a flat valley configuration for a Fresnel lensaccording to an embodiment of the present disclosure.

FIG. 23 is a plot of light patterns for a fist pixel where the lightpatterns are produced using a Fresnel lens of a first configuration.

FIG. 24 is a plot of light patterns for a fist pixel where the lightpatterns are produced using a Fresnel lens of a second configuration.

FIG. 25 a plot of light patterns for a second pixel where the lightpatterns are produced using a Fresnel lens of a first configuration.

FIG. 26 is a plot of light patterns for a second pixel where the lightpatterns are produced using a Fresnel lens of a second configuration.

FIG. 27 is another illustration of the flat valley configuration of FIG.22.

FIG. 28 provides an exemplary illustration between the depth ratio ofthe Fresnel lens and the radial distance from the center of the Fresnellens according to an embodiment of the present disclosure.

FIG. 29 provides an exemplary illustration between the depth of valley,ray and mold as a ratio to pitch of a Fresnel lens configuration as afunction of the radial distance from the center of the Fresnel lens.

DETAILED DESCRIPTION

An ultra-thin rear projection display system is described. In thefollowing description, for purposes of explanation, numerous specificdetails are set forth in order to provide a thorough understanding ofthe invention. It will be apparent, however, to one skilled in the artthat the invention can be practiced without these specific details. Inother instances, structures and devices are shown in block diagram formin order to avoid obscuring the invention.

The ultra-thin rear projection display device described herein includesa wide-angle lens system and one or more planar mirrors that areparallel to a screen on which an image is to be displayed. In oneembodiment, the screen that has multiple groove angles to provide betterillumination than screens with a single groove angle. As described ingreater detail below, the screen may be a Fresnel lens having one ormore groove angles.

FIG. 3 illustrates one embodiment of an ultra-thin rear projectiondisplay device with planar mirrors parallel to a screen. As described ingreater detail below, use of planar mirrors parallel to the screen, aswell as, a wide angle projection lens having an optic axis that isperpendicular to the mirrors and the screen allows the ultra-thin rearprojection display device to be thinner and simpler than prior art rearprojection display devices. For example, an ultra-thin rear projectiondisplay device, as described herein, which is less than 7 inches thickmay produce a 60-inch image.

In one embodiment, ultra-thin rear projection display device 300includes screen 310, back plate mirror 320, intermediate mirror 330,lens system 340 and digital micromirror device (DMD) 350. Othercomponents, for example, image generating components are not illustratedfor reasons of simplicity of description. An image can be provided toDMD 350 in any manner known in the art. DMD 350 selectively reflectslight from a light source (not shown in FIG. 3) to lens system 340.Other types of devices (e.g., microelectromechanical systems (MEMS),grating light valve (GLV), liquid crystal display (LCD), liquid crystalon silicon (LCOS)) can be used to provide an image to lens system 340.In one embodiment, the mirrors may be substantially parallel to thescreen, which implies an alignment error of +/−10°. In one embodiment,the optic axis of the wide-angle lens system may be substantiallyperpendicular to the screen, which also implies an alignment error of+/−10°.

In one embodiment, DMD 350 is offset from the optic axis of lens system340 such that only a portion (e.g., 50%, 60%, 40%) of the available lensfield is used. The image from DMD 350 may be projected by lens system340 in the upper portion of the lens field to intermediate mirror 330,in an embodiment of the invention. The image may then be reflected toback plate mirror 320 and finally to screen 310.

In an alternative embodiment of the invention, the image from DMD 350 isprojected by lens system 340 in the lower portion of the lens field tointermediate mirror 330. In such an embodiment, wide-angle lens system340 may be, at least partly, above intermediate mirror 330. Intermediatemirror 330, in turn, may be, at least partly above back plate mirror320. The image is then reflected to back plate mirror 320 and finally toscreen 310.

In order to project an image as described, lens system 340 may be a verywide-angle lens system. In one embodiment, lens system 340 has a fieldangle of 152° or more; however, other lenses may be used. In general,the wider the angle of lens system 340, the thinner display device 300can be made. Description of a suitable wide-angle lens system isdescribed in U.S. patent application Ser. No. 10/222,050 entitled WideAngle Lens System Having a Distorted Intermediate Image, filed Aug. 16,2002, which is hereby incorporated by reference.

Intermediate mirror 330 reflects the image to back plate mirror 320,which reflects the image to screen 310. In one embodiment, screen 310 isa Fresnel lens. Back plate mirror 320 is also a planar mirror and isparallel to screen 310 and perpendicular to the optic axis of lenssystem 340. Because the optic axis of lens system 340 is perpendicularto intermediate mirror 330 and both intermediate mirror 330 and backplate mirror 320 are planar and parallel to screen 310, the distortioncaused by angled lenses and aspherical mirrors is absent in displaydevice 300. This simplifies the design of display device 300 and reducesthe cost and complexity of manufacturing.

FIG. 4 illustrates a Fresnel lens with outlines of sections to be usedfor rear projection screens. FIG. 4 provides a conceptual illustrationof the sections of a Fresnel lens that can be used for various rearprojection display devices. The Fresnel lens can be described with twoangles. The face angle is defined as the angle of the surface of eachindividual groove through which light passes as it enters, or in thecase of some refractive designs exits the Fresnel lens relative to theoptic axis of the lens. The groove angle is the angle formed between theinput face and the reflection face, or in the case of a refractive lensbetween the optical face of the groove and the non-optical face. Faceangles and groove angles are more fully discussed below with referenceto FIG. 16.

In one embodiment, Fresnel lens 400 can have many concentric grooveshaving one or more predetermined groove angles. Techniques formanufacturing and using Fresnel lenses having a single groove angle areknown in the art. In a rear projection display device in which the fulllens field of the projection lens system is used, a center portion 420of Fresnel lens 400 is used for the lens of the display device.

Dashed rectangle 420 provides an indication of a screen from the centerportion of Fresnel lens 400. The size and shape of the portion of thelens to be used corresponds to the size and shape of the screen of thedisplay device. For example, in some rear projection displays, thecenter of section 420 may be used for a screen, which is the center ofFresnel lens 420.

When using an offset DMD (or other device) so that only a portion of theprojection lens field is used, the section of Fresnel lens 400 used fora screen is correspondingly offset from the center of Fresnel lens 400.For example, if the top half of the projection lens field is used, thebottom edge of screen portion 410 passes through the center of Fresnellens 400.

FIG. 5 illustrates a cross-sectional profile view of a Fresnel lens 500having a groove angle 510 of 39°. The lens of FIG. 5 can be used with,for example, the display system of FIG. 3. When used with a displaysystem as illustrated in FIG. 3 with an offset as described with respectto FIG. 4, the groove angle of 39° provides a balance between diamondcutter structural integrity and lens performance.

As the groove angle increases the image projected to the bottom centerof lens 500 becomes dark because rays pass through the lens withoutbeing reflected from the intended total internal reflection (TIR)surface on the exterior of the groove. As the groove angle decreases,the image projected to the top corners of lens 500 become dark becausereflected rays are directed down and away from the viewer. Also, as thegroove angle decreases, the tool used to manufacture lens 500 can becometoo weak to work effectively.

FIG. 6 a illustrates a front view of a Fresnel lens having two zoneseach having a different groove angle. The embodiment of FIG. 6 aillustrates two zones with two groove angles; however, any number ofzones with corresponding groove angles can be used. The groove angle ofa lens can vary continuously. Also, while the example of FIG. 6 aillustrates circular regions, other shapes can also be used.

In one embodiment, interior region 620 has grooves of approximately 35°;however, other groove angles can also be used. When used for largescreens, a Fresnel lens with a single groove angle throughout providesnon-uniform illumination. In one embodiment, outer region 610 hasgrooves of approximately 410; however, other groove angles can also beused. In alternate embodiments, interior region 620 and outer region 610can provide any combination of refraction and/or reflection lenses. Inone embodiment, the projector side of lens 600 has grooves and theviewer side is planar. In an alternate embodiment, lens 600 has grooveson both sides.

FIG. 6 b illustrates a cross-sectional profile view of a two-zoneFresnel lens having a first zone with a groove angle of 35° and a secondzone having a groove angle of 41°. The lens of FIG. 6 b can be usedwith, for example, the display system of FIG. 3. The lens of FIG. 6 bmay provide improved uniformity as compared to the lens of FIG. 5.

In one embodiment, the grooves of zone 620 provide a refractive lens andthe grooves of zone 610 provide a total internal reflection (TIR) lens.The refractive and reflective zones of lens 600 can be on the same sideof the lens (e.g., the projector side) or the refractive and reflectivezones of lens 600 can be on opposite sides (e.g., reflective on theprojector side and refractive on the viewer side). An example of anembodiment of the invention wherein the refractive and reflective zonesof a lens are on opposite sides is described below with reference toFIG. 10. As described in greater detail below, transition regions can beused to reduce or even eliminate image artifacts caused by transitionsbetween zones. For a double-sided lens, two single-sided lenses can bealigned and the planar sides of each lens can be bonded together.Alternatively, one side of the lens may be manufactured, for example, bya curing process and additional grooves can be formed directly on theopposite side of the lens by the same process.

FIG. 7 illustrates an input ray having a 600 input angle with a Fresnellens having a face angle of 100. For steep angles of input light (e.g.,greater than about 45°) it is possible to design face angles of thegrooves such that all light enters the Fresnel lens and reflects off ofreflection face and travels directly toward the viewer. For example,input light 720 passes through groove face 700 and is slightlyrefracted. Refracted light 730 is reflected by reflection face 710toward a viewer (not shown in FIG. 7). For most applications, reflectedlight 740 is directed toward the viewer.

As the angle of the input light decreases, there is an angle at whichthe refracted light misses reflection face 710. This occurs, forexample, at the bottom center of the screen at the grooves closest tothe Fresnel center. This light is lost and travels through the Fresnelstructure creating either a ghost image or a reduction in contrast. Thelost light reduces contrast at the bottom center of the screen area (andpossibly everywhere depending on where the mirrors are with respect tothe screen).

One technique to reduce ghost rays and improve contrast in these areasis to change the reflection face angle such that, instead of directinglight toward the viewer, the lens is designed to collect as much lightas possible. As a consequence, the reflected light ray 740 travelsdownward. This improves the contrast of the displayed image, but thedownward light does not get redirected to viewer as well and appearsdark.

The face angles can be designed so that light from the top corners ofthe screen, where the input rays are steep, is reflected slightly towardthe center of the lens to improve perceived brightness at the corners ofthe image. An example of an embodiment of the invention in which lightfrom the top corners of the screen is reflected toward the center of thelens is more fully described below with reference to Table 1, Equation2, Table 2, and FIG. 17.

FIG. 8 illustrates a profile cross-sectional view of a Fresnel lensdesign having two zones with different groove angles and a transitionregion between the zones. Lens 800 is illustrated with only a smallnumber of grooves, zones and regions. This is for simplicity ofdescription. A Fresnel lens may be used that has any number of grooves,zones, and/or regions.

As used herein, a “zone” is an area of a Fresnel lens having aparticular groove angle (when the groove angle is not continuouslyvariable). A “region” is an area of a Fresnel lens in which the faceangle (γ) is defined by a single equation. A zone may include multipleregions. In one embodiment, one or more transition regions are includedat zone boundaries in order to provide a smooth zone transition.

In one embodiment, the equation, F, that defines the face angle, whichcan be a function of radius, r, for a first region and the equation, G,that defines the face angle for a second region, are equal at the regionboundary. In other words, F(r₁)=G(r₁) where r₁ is the region boundary.Further, the first derivative of the equation that defines the faceangle for a region is equal to the first derivative of equation thatdefines the face angle at the region boundary. In other words,F′(r₁)=G′(r₁) where r₁ is the region boundary. This requirement providesfor a transition that is not seen because the change in face angle issmoothly continuous.

FIG. 9 illustrates a profile cross-sectional view of a Fresnel lensdesign. In one embodiment, the following equations describe the variousangles of the Fresnel lens design. Alternative angle relationships alsomay be used. In the equations that follow, θ6 is the input angle, or theangle of input ray 920 from horizontal; γ is the face angle, or theangle of refraction face 910 from horizontal; δ is the reflection faceangle, or the angle of reflection face 900 from horizontal; ρ is therefracted ray angle, or the angle of refracted ray 930 from horizontal;θ2 is the reflected ray angle, or the angle of reflected ray 950 fromhorizontal; and β is the output ray angle, or the angle of output ray960 from horizontal.

In one embodiment, the following equations are used to determine theangles to be used for various regions. For a fixed peak angle (peakangle k=γ+δ), the face angle can be calculated to create a Fresnel lenswith no ghost rays near the bottom center and the face angles aremodified to increase throughput.

For a two region embodiment, the inner region can be a lossless systemdefined by:${{F\left( {R,\gamma} \right)}\text{:}} = \left\lbrack {\frac{\begin{matrix}{{{\tan(\gamma)} \cdot \left( {{\tan(\gamma)} + {2 \cdot {\tan\left( {k - \gamma} \right)}}} \right)} + \tan} \\{\left( {\frac{\pi}{2} - \gamma - {{asin}\left( \frac{\cos\left( {{{atan}\left( \frac{R}{fl} \right)} + \gamma} \right)}{n} \right)}} \right) \cdot {\tan\left( {k - \gamma} \right)}}\end{matrix}}{{\tan\left( {\frac{\pi}{2} - \gamma - {{asin}\left( \frac{\cos\left( {{{atan}\left( \frac{R}{fl} \right)} + \gamma} \right)}{n} \right)}} \right)} - {\tan\left( {k - \gamma} \right)}} - \frac{R}{fl}} \right\rbrack$

where n is the refractive index of the Fresnel lens material, k is thegroove angle, R is the radius from the center of the Fresnel lens, andfl is the focal length of the Fresnel lens.

Outer regions are defined by:${F\quad 2\left( {R,\gamma} \right):} = {\frac{\pi}{2} - \gamma - {{asin}\left( \frac{\cos\left( {{{atan}\left( \frac{R}{fl} \right)} + \gamma} \right)}{n} \right)} - {2\left( {k - \gamma} \right)} - {\theta\quad 2}}$

FIG. 10 illustrates an embodiment of a Fresnel lens having two zoneswith grooves on both sides of the lens and a transition region for thetwo zones. Fresnel lens 1090 includes two zones: a refractive zone and areflective zone, as well as a transition region between the two zones.In alternate embodiments, lens 1090 can have one or more zones on asingle side.

In one embodiment, Fresnel lens 1090 includes an inner zone that is aconventional refractive Fresnel lens design 1000. The inner zone mayinclude the center of lens 1090 extending outward until the outer zonebecomes more efficient than the inner zone. Fresnel lens 1090 furtherincludes an outer zone that is a total internal reflection Fresneldesign 1020. The outer zone directs more light toward the viewer than ifthe refractive design of the inner zone were to extend to the edge ofthe lens.

In order to reduce, or even eliminate, discontinuities between therefractive and the reflective portions of lens 1090, transition region1010 is included. In one embodiment, in transition region 1010, thelight rays internal to Fresnel lens 1090 change gradually from theupward angle of the refractive design to the horizontal angle of thereflective design. The gradual change reduces image discontinuities dueto overlapping rays.

FIG. 11 illustrates one embodiment of a rear projection display devicehaving a wide-angle lens. Display device 1100 includes screen 1110,wide-angle lens system 1120 and DMD 1130. In one embodiment, screen 1110is a Fresnel lens as described in greater detail above.

An image may be generated by optical engine components (not shown inFIG. 11) that are known in the art and directed to wide-angle lenssystem 1120 via DMD 1130. In some embodiments, DMD 1130 may be replacedby other components, for example, microelectromechanical systems (MEMS),grating light valves (GLV), liquid crystal display (LCD), liquid crystalon silicon (LCOS), etc. In one embodiment, the optic axis of DMD 1130 isaligned with the optic axis of wide-angle lens system 1120 so that thefull lens field is used to project the image to screen 1110. Inalternate embodiments, the optic axis of DMD 130 can be offset from theoptic axis of wide-angle lens system 1120. Use of a Fresnel lens, asdescribed above, may provide a thinner system with better brightnessuniformity.

Diffusing Stray Light

FIG. 12 illustrates rear projection display device 1200 and thepotential problem of stray light. Rear projection display device 1200includes screen 1210, back plate mirror 1220, intermediate mirror 1230,wide-angle lens system 1240, and digital micromirror device (DMD) 1250.DMD 1250 and wide-angle lens system 1240 project an image ontointermediate mirror 1230. Intermediate mirror 1230 reflects theprojected image to back plate mirror 1220. Light reflected from backplate mirror 1220 may produce a desired image (e.g., ray 1254) and anundesirable image (e.g., stray rays 1258, 1260, and 1262). For example,if light travels the path defined by reference numeral 1252, it mayproduce desired ray 1254.

The angular surfaces of screen 1210 (e.g., the flat output surface) actas fairly good mirrors and coherently reflect some of the light thatimpinges on the surfaces. Light that is coherently reflected from theangular surfaces of screen 1210 may produce objectionable stray images.For example, light may travel the path defined by reference numerals1252, 1266, 1268, and 1270 to produce stray ray 1258. Similarly, lightmay travel the path defined by 1252 and 1274 to produce stray ray 1262.A third example of the path “stray light” may take is shown by referencenumerals 1252, 1276, 1278, and 1280 to produce stray ray 1260. A personof ordinary skill in the art appreciates that stray images may beproduced by light traveling paths other than the exemplary paths shownin FIG. 12.

FIG. 13 illustrates rear projection display device 1300, with elementsto reduce stray images. Rear projection display device 1300 may includemore components than those shown in FIG. 13 or a subset of thecomponents shown in FIG. 13. It is not necessary, however, that all ofthese generally conventional components be shown in order to disclosestray light reduction.

In one embodiment, rear projection display device 1300 includes Fresnellens 1310, back plate mirror 1320, intermediate mirror 1330, wide-anglelens system 1340, and digital micromirror device (DMD) 1350. Othercomponents, for example, image generating components are not illustratedfor reasons of simplicity of description. Fresnel lens also may includebumps 1370, diffuser 1380, and/or diffusion layer 1390.

Bumps 1370 help to reduce stray light visibility by scattering the straylight in many different directions. In some embodiments, bumps 1370 areaffixed to the output side of Fresnel lens 1310. In alternativeembodiments, bumps 1370 are formed on the surface of (e.g., are ofunitary construction with) Fresnel lens 1310. In such embodiments, bumps1370 may be formed by a curing process (e.g., an ultra violet (UV)curing process). Curing processes, including UV curing processes, arewell known in the art. In yet other alternative embodiments, bumps 1370may be formed by abrading a surface of Fresnel lens 1310 (e.g., abradingthe output surface of Fresnel lens 1310).

Bumps 1370 are typically formed from translucent materials such asplastic or glass. In some embodiments, bumps 1370 are formed from thesame material as Fresnel lens 1310. In alternative embodiments, bumps1370 are formed from a different material than the material used to formFresnel lens 1310.

In an embodiment, bumps 1370 are lenticular bumps. The term lenticularbump broadly refers to a bump having a convex cylinder shape. Inalternative embodiments, bumps 1370 are two-dimensional hills that areregularly or randomly distributed across the output side of Fresnel lens1310. In an embodiment, at least one bump 1370 (e.g., 1370A) has adifferent size and/or shape than another bump (e.g., 1370B).

Fresnel lens 1310 may include diffuser 1380 to reduce stray light.Diffuser 1380 is typically formed from a translucent material such asplastic or glass. In an embodiment of invention, diffuser 1380 is formedby adding beads (e.g., white and/or tinted beads) to the material fromwhich Fresnel lens 1310 is formed, while that material is in a liquidstate. In such an embodiment, diffuser 1380 is said to be “of unitaryconstruction with” Fresnel lens 1310.

The optical qualities of diffuser 1380 may be carefully selected so thatlight passing through diffuser 1380 a single time is not significantlyaltered. In contrast, light passing through diffuser 1380 multiple timesis scattered in many directions to reduce the likelihood that it willinterfere with the image quality of ultra-thin rear projection displaydevice 1300.

Diffusion layer 1390 provides an alternative (and/or complimentary)mechanism for reducing stray light in an embodiment of the invention.The characteristics of diffusion layer 1390 are similar to those ofdiffuser 1380. For example, diffusion layer 1390 is typically formedfrom a translucent material designed to scatter light that passesthrough it more than once. Since diffusion layer 1390 is thin and closeto the image surface, stray light is diffused without significantlyreducing the sharpness of a displayed image.

Diffusion layer 1390 is affixed to the output surface of Fresnel lens1310, in an embodiment. In alternative embodiments, diffusion layer 1390is formed in a curing process (e.g., UV curing) on a surface of Fresnellens 1310. In an exemplary embodiment, diffusion layer 1390 isapproximately 0.8 millimeters thick (+/−10 percent). In alternativeembodiments, diffusion layer 1390 may be thinner or thicker than 0.8millimeters and may have a different tolerance (e.g., +/−3%, +/−5%,+/−12%, +/−15%, etc.)

In an embodiment, Fresnel lens 1310 includes one of diffuser 1380,diffusion layer 1390, and bumps 1370. In an alternative embodiment,Fresnel lens 1310 includes a combination of diffuser 1380, diffusionlayer 1390, and/or bumps 1370. Fresnel lens 1310 may include anycombination of diffuser 1380, diffusion layer 1390, and/or bumps 1370.

FIG. 14 illustrates an exemplary bump 1410 scattering light rather thancoherently reflecting light. Rays 1420, 1430, and 1440 are substantiallyparallel to each other and impinge on bump 1410. If rays 1420, 1430, and1440 impinge on a flat surface they may be reflected coherently withrespect to one another and are more likely, therefore, to create a strayimage. Since the surface of bump 1410 is curved, however, each ray has adifferent angle of incidence with respect to bump 1410. Therefore, bump1410 scatters rays 1420, 1430, and 1440. Rays 1450, 1460, and 1470illustrate the scattering effect of bump 1410.

FIG. 15 illustrates an exemplary diffusion layer 1510 diffusing lightthat might otherwise form a stray image. Ray 1520 impinges on Fresnellens 1530. Ray 1520 travels through diffusion layer 1510 and is diffusedslightly into rays 1540, 1542, and 1544. Light from rays 1540, 1542, and1544 may reflect off of the flat output surface of Fresnel lens 1530.

Ray 1550 is an exemplary ray reflecting off of the flat output surfaceof Fresnel lens 1530. Ray 1550 travels through diffusion layer 1510 andis diffused into rays 1560, 1562, and 1564. If rays 1560, 1562, and 1564return to Fresnel lens 1530 they are widely separated and will not forma visible stray image.

Exemplary Fresnel Equation

FIG. 16 illustrates the relationship between face angle (γ) and outputray angle (β), according to an embodiment of the invention. As shown inFIG. 16, input light 1610 reaches Fresnel lens 1600 with an input rayangle theta (θ). The groove angle for the illustrated zone of Fresnellens 1600 is shown by angle lambda (λ). As previously discussed inconnection with FIG. 6 a through FIG. 8, Fresnel lens 1600 may have morethan one zone and each zone may have a different groove angle. Thevarious zones of Fresnel lens 1600 may be defined by distances (R) fromthe center of the Fresnel lens (e.g., the center of Fresnel lens 600,shown in FIG. 6 a). Table 1 provides a zone equation for the illustratedembodiment of the invention. The zone equation expresses face angle (γ)in terms of the refraction angle, output ray angle (β), and groove angle(λ). TABLE 1 First zone Minimum radius R = 245 Maximum radius R = 1230Zone equation$\gamma = {{\tan^{- 1}\left( \frac{{n\quad\sin\left\{ {\beta^{\prime} + \lambda} \right\}} + {\sin\left( {\theta + \lambda} \right)}}{{n\quad\cos\left\{ {\beta^{\prime} + \lambda} \right\}} - {\cos\left( {\theta + \lambda} \right)}} \right)} + \lambda - \frac{\pi}{2}}$Equation coefficients n = 1.55 β′ = sin⁻¹ (sin β/n) Groove angle (λ) 38°

Equation 2 describes how output ray angle (β) varies with the radialdistance R, in an embodiment of the invention. Equation 2 is expressedas a spline equation. Spline equations are well known to those ofordinary skill in the art. $\begin{matrix}{{{\beta = {\beta_{1} + {\sum\limits_{k = 1}^{4}{\Delta_{k}\left\lbrack {\left\{ {1 + \left( {1 + \frac{R - R_{0}}{R_{5} - R_{0}} - \frac{R_{k} - R_{0}}{R_{5} - R_{0}}} \right)^{m}} \right\}^{\frac{1}{m}} - 1} \right\rbrack}}}},{where}}{\Delta_{1} = {\frac{\beta_{2} - \beta_{1}}{\frac{R_{2} - R_{0}}{R_{5} - R_{0}} - \frac{R_{1} - R_{0}}{R_{5} - R_{0}}}\quad{and}}}\text{}{\Delta_{\quad k} = {\frac{\beta_{k + 1} - \beta_{k}}{\frac{R_{k + 1} - R_{0}}{R_{5} - R_{0}} - \frac{R_{k} - R_{0}}{R_{5} - R_{0}}} - {\frac{\beta_{k} - \beta_{k - 1}}{\frac{R_{k} - R_{0}}{R_{5} - R_{0}} - \frac{R_{k - 1} - R_{0}}{R_{5} - R_{0}}}.}}}} & {{Equation}\quad 2}\end{matrix}$

Table 2 provides the coefficients for equation 2 in an exemplaryembodiment of the invention where m is 16 and R₀ is 230 millimeters.TABLE 2 k 1 2 3 4 5 R [mm] 260 300 650 950 1232 β [°] 0 0 0 5.5 8.5

FIG. 17 provides an exemplary illustration of face angle (γ) and outputray angle (β) varying as a function of radial distance (R) from thecenter of a Fresnel lens, according to an embodiment of the invention.As illustrated in FIG. 17, face angle (γ) is nonzero in a region closeto the center of the Fresnel lens and approaches zero as the radialdistance from the center of the Fresnel lens increases. In contrast,output ray angle (β) is nearly zero for small values of the radialdistance R and increases as the value of R increases. Thus, in theillustrated embodiment output ray angle (β) is substantially close tozero (e.g., +/−10°) for values of R corresponding to the center of theFresnel lens and increases for values of R corresponding to the cornersof the Fresnel lens. In alternative embodiments, the relationshipsbetween face angle (γ), output ray angle (β), and radial distance fromthe center of a Fresnel lens (R) may be different than those illustratedin FIG. 17.

The Relationship Between The Screen Diagonal Length And The FocalDistance Of The Fresnel Lens

FIG. 18 is a front view of screen 1800, such as a Fresnel screen.Reference numeral 1810 illustrates the screen diagonal of screen 1800.Screen diagonal refers to the distance from one corner of screen 1800 tothe opposite corner of the screen. In an embodiment, the screen diagonalmay be the diagonal length of the viewable portion of screen 1800. In analternative embodiment, the screen diagonal may be the diagonal lengthof the actual size of screen 1800.

Reference numerals 1820 and 1830, respectively, illustrate the width andheight of screen 1800. The ratio of width 1820 to height 1830 definesthe aspect ratio of screen 1800. In an embodiment, the aspect ratio ofscreen 1800 is 16:9. In an alternative embodiment, the aspect ratio ofscreen 1800 is 4:3. Screen 1800 may have an aspect ratio other than 16:9and 4:3.

FIG. 19 illustrates rear projection display device 1900. Rear projectiondisplay device 1900 includes wide-angle lens system 1910 and screen1920. In an embodiment, screen 1920 is a Fresnel lens. Focal distance1930 represents the focal length of Fresnel lens 1920. The term focallength refers to the distance from the optical center of Fresnel lens1920 to focal point 1940. The term focal point may refer to the spot atwhich impinging rays converge to a common point or focus. Aberrated raysmay also form a focal point. The term “circle of least confusion” refersto a focal point formed by aberrated rays. The focal point is usuallyplaced near the pupil of a projection lens (e.g, the pupil of theprojection lens of wide-angle lens system 1910.

Focal distance 1930 may be used to express the thinness of rearprojection display device 1900. For example, the thinness of rearprojection display device 1900 may be expressed by the ratio of thescreen diagonal of Fresnel lens 1920 to focal distance 1930. In anembodiment in which the screen diagonal is 60 inches, the ratio of thescreen diagonal to focal distance 1930 is approximately 3.0. In analternative embodiment of the invention in which the screen diagonal is70 inches, the ratio of screen diagonal to Fresnel focal distance isapproximately 4.1.

Alternative Fresnel Lens Configuration

Various configurations of Fresnel lens, diffusion layers, bumps, etc.may be used to reduce stray light. FIG. 20, similar to FIG. 12,illustrates schematically the production of stray light that may occuras image light is directed through a Fresnel lens 2000. Specifically, inFIG. 20, image ray 2010 may be directed along light path 2020 to producea desired image (also referred to as main image). However, undesiredlight, such as scattered light or stray rays 2030 may be generatedduring production of a desired image. The stray light may producevisible artifacts which may be undesirable when viewing an image. Forexample, there may be ghost images, flair and other stray light that isspaced from the desired image pixel disrupting the clarity of the pixeland the surrounding pixels.

There are many causes for such stray light. For example, stray ray 2030may be caused by a surface reflection (shown at 2050) off of a grooveface in Fresnel lens 2000. Such reflection off of the groove face mayresult in the image being spread out due to the extra light produced inclose proximity to the desired pixel.

Other stray light may be produced. Such stray light may be generated byreflecting off of the surface of a groove (as again shown at 2050). Somestray light may have one or more additional surface reflections off ofvarious grooves and groove surfaces. In some situations, the light maytotally internally reflect (TIR) off the front surface 2060 and thenhave additional surface reflections off of more groove surfaces of theFresnel lens.

FIG. 21 illustrates schematically stray light paths that may result inthe production of ghost images. As with FIG. 20, image ray 2010 and itsrespective light path 2020 is exemplary of main image light configuredto produce a desired image. Stray rays 2070 and 2080 schematicallyillustrate rays which may create ghost images. Ghost images may occurwhere light exits above or below the desired image. For example, ghostimages may occur where light exits three or four pixels above or belowthe intended pixel. Such ghost images may produce visible artifacts,such as displaced replica of a pixel or image. As with the other strayrays, the ghost rays may be caused by internal and surface reflectionalong the grooves of the Fresnel lens structure.

It should be understood that the light paths shown in FIGS. 20 and 21are for illustrative purposes only, and the light paths (both stray andmain image light paths) may vary without departing from the scope of thedisclosure.

FIG. 22 illustrates a method for reducing the visibility of stray lightgenerated by the Fresnel lens. As shown in FIG. 22, the Fresnel lens maybe configured such that the majority of light that would produce strayimages is scattered. For example, in FIG. 22, the Fresnel lens ismodified such that the valleys of the Fresnel lens are flattened asindicated at 2200. For purposes of discussion, a valley is shown in itsoriginal configuration in dashed lines at 2210. In the newconfiguration, the valley includes a valley floor 2220. As used herein,a valley floor includes any flattening and/or leveling of theintersection between two grooves. The original peak and valleyconfiguration (shown in FIGS. 20 and 21) is not considered to have avalley floor.

The flattening of the valley to produce a valley floor may operate toremove the portion of the groove surfaces that the stray rays previouslyreflected off of. Typically, the main image 2010 follows a light path2020 that uses the peak of the Fresnel grooves. In contrast, the strayrays use the valley portion of the grooves, such as via surfacereflection, to produce ghost images, flairs, etc. By reducing thesurface from which the stray rays reflect, it is possible to reduce theamount of concentrated stray light. In other words, the stray lighttypically follows stray light paths which utilize the valleys. Byremoving the valleys of the Fresnel lens, the stray light pathways maybe disrupted and the stray light rays scattered.

It should be appreciated that the use of the flat valley Fresnel lensmay produce stray light along new stray light pathways. However, much ofthis light is scattered by the valley floor 2220. For example, stray ray2230 has a surface reflection at 2240 off of the face of a groove. Strayray 2230 then is directed towards the valley floor. The valley floorresults in the scattering 2250 of stray light 2230. The scatteringeffect diminishes the visible effect of the stray ray.

In some embodiments, the Fresnel lens configuration shown in FIG. 22,may be considered to include units having a first surface configured toreceive an image ray, where the image ray may be configured to impinge atip portion of the first surface. A second surface may face the firstsurface and be linked to the tip portion of the first surface through athird surface, or valley floor. The third surface may be configured toscatter stray light reflected from the first surface. It should beappreciated that the flat valley may be smooth (no diffuser) and stillhave some effect. However, in some embodiments, the flat valley may berough such that it scatters additional light. Thus, in some embodiments,the valley floor may be considered a scattering floor.

In some embodiments, the Fresnel lens may lie substantially within aplane with the valley floor extending substantially parallel to theplane of the Fresnel lens, such that the valley floor is flat. Althoughthe valley floor is disclosed as being flat, it should be appreciatedthat in some embodiments the valley floor may include surface topographysuch as ridges, bumps, elevations and/or depressions. The ridges orbumps may operate to increase the scattering effect of the stray rays.Moreover, the valley floor may be inclined or sloped in someembodiments. In other embodiments, the valley floor may be blackened orotherwise textured to absorb the stray light and/or substantiallydisperse the stray light.

Many of the stray rays shown in FIGS. 20 and 21 may be substantiallyeliminated or greatly diminished by use of the flat valley configurationshown in FIG. 22. Specifically, the flat valley configuration may alterthe stray light patterns and or diminish the intensity of the stray raysby scattering the light.

FIGS. 23-26 provide simplified contour plots with exemplary shading ofthe light pattern intensity for a selected pixel of a Fresnel screen.While it is possible to produce a contour plot illustrating relativeintensities of the light pattern, for the purposes of the presentdescription only the relative locations of the light needs to beillustrated in FIGS. 23-26.

FIG. 23 illustrates an upper pixel of a Fresnel screen having theoriginal peak and valley configuration. The left side of FIG. 23 is anall-light plot including both the main image light and any stray light.The main image is indicated at 2310, however other light (stray light)such as 2320 may be visible. Such stray light may affect the quality ofthe image at the pixel.

The stray light pattern is more visible in the right side plot 2330 ofFIG. 23 where the main image light has been removed. It should be notedthat the intensity of some stray light (such as stray light 2320) issignificant during production of the image to cause undesirable visualeffect to a viewer.

FIG. 24 illustrates the same upper pixel as shown in FIG. 23 but with aflat valley Fresnel lens configuration. As with FIG. 23, the left sideof FIG. 24, is an all-light plot (main image light and stray light) andthe right side of FIG. 24 is a stray-light only plot. Comparing theall-light plots of FIGS. 23 and 24, a significant difference may berecognized in the amount of visible stray light localized near the imagepixel. Moreover, the stray-light only plot in FIG. 24 shows that thelight is scattered much more broadly (indicated at 242) and isconsequently less visible to a viewer. The more scattering of the straylight, the more attenuated any effects the stray light may have on thevisible image.

FIGS. 25 and 26 further illustrate the effect of the flat valleyconfiguration over the original peak and valley configuration.Specifically, FIGS. 25 and 26 illustrate a pixel localized in the middleof the screen. As shown in FIG. 25, the main image 2510 may be flankedby other intense light spots, such as ghost images 2520.

FIG. 26 illustrates the diminishing effect of the flat valleyconfiguration on the ghost image. Specifically, the all-light plot showsa substantial amount of light localized at the desired image spot. Theghost light has been reduced significantly. Specifically, the ghostimage shown in FIG. 25 was approximately 1.0% of the peak intensity ofthe pixel. The ghost image in FIG. 26 is only 0.3% of the peak intensityof the pixel. This significant reduction in intensity results inminimization of the visibility of the ghost images to a viewer.

It should be noted that the stray-light only plot in FIG. 26 furtherillustrates the scattering of the stray light. The more scattered thestray light, the less effect the stray light has on the desired image.Thus, although some stray rays may follow identical or similar paths asoccurred in the original peak and valley configuration, the amount oflight that travels such paths may be substantially decreased in the flatvalley configuration, thus minimizing the stray rays effects on thedisplayed image.

FIG. 27 illustrates the flat valley configuration in more detail.Although many suitable methods may be used to determine the desiredvalley floor depth, one exemplary method is described below. It shouldbe appreciated that other methods may be used to determine the valleyfloor depth. Moreover, the valley floor depth may vary from valley tovalley depending on the input angle of the main image light.

As shown in FIG. 27, the Fresnel lens may include a first sloped surfaceangled to receive input light (such as light as 2710). A second slopedsurface may face the first sloped surface. A valley floor may link thefirst sloped surface to the second sloped surface. As described above,the valley floor may be configured to scatter stray light reflected fromthe first sloped surface.

The combination of a first sloped surface, a second sloped surface and avalley floor create a light input unit. Multiple light input units maybe linked together to form the configuration shown in FIG. 22. In someembodiments, neighboring light input units may vary from each other. Forexample, the depth of the valley floor may vary from light input unit tolight input unit or zone of light input units to zone of light inputunits. Moreover, in some embodiments, the light input units may beinterspersed with units that include a first sloped surface directlyconverging with a second sloped surface.

For the purposes of the exemplary method, a useful ray depth, indicatedat 2720, may be determined using an input ray, such as ray 2710. Theuseful ray depth may vary relative to the input ray angle. In theillustrated embodiment, the ray depth may be used to determine the depthof the valley floor at 2730. It should be appreciated that othersuitable methods may be used to determine the depth of the valley floor.In the exemplary figure, a depth ratio may be calculated as follows:Depth Ratio=Useful Ray Depth/Groove Depth.

Briefly, in some embodiments, the valley floor depth may be based on auseful input ray angle. A useful input ray angle may be the angle wherean image ray directed through a first sloped surface is configured togenerate an image at a desired pixel, as illustrated by input ray 2710.

FIG. 28 shows an exemplary illustration of the relationship between theDepth Ratio (described above) and the radial distance (R) from thecenter of a Fresnel lens. R may be considered the Fresnel radius. Asillustrated, the Depth Ratio decreases as the radial distance from thecenter of the Fresnel lens increases. It should be appreciated thatflattening the valley floor for small values of the radial distance maybe ineffective because the light may miss the TIR surface for such smallvalues, and thus, any valley flattening may not cause any significantimprovement. When the depth ratio is greater than 1.0, the light maymiss the TIR surface, which may cause loss of light.

FIG. 29 further provides an exemplary illustration of the relationshipof ray (at 2910), depth (at 2920) and mold (at 2930) of a Fresnel lensconfiguration as a ratio to pitch as a function of the radial distance(R) from the center of a Fresnel lens. In the illustrated embodiment,Ray 2910 is equal to Useful Ray Depth/Pitch and decreases as the radialdistance increases. Depth 2920 (Original Valley Depth/Pitch) maydecrease only slightly, remaining relatively constant as the radialdistance increases. Mold 2930 (Flat Valley Floor Depth/Pitch) also maydecrease as the radial distance increases. It should be appreciated thatthe relationship between depth ratio and radial distance, as well as therelationships between ray, depth and mold and radial distance, may bedifferent in alternative embodiments, and the descriptions providedherein are provided for illustrative purposes only.

The flat valley configuration may be used in combination with diffusersor other structures configured to reduce and/or diffuse stray light.Thus, it should be appreciated that the embodiments, in whole or inpart, throughout the disclosure may be combined with the flat valleyconfiguration.

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the invention. The appearances of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes can be made thereto withoutdeparting from the broader spirit and scope of the invention. Thespecification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

1. A Fresnel lens for a display device, the Fresnel lens comprising: an input side configured to receive an image from an image projecting assembly, the input side including: a first facet angled to receive input light constituting at least a portion of the image, a second facet at least partially facing the first facet, and a third facet linking the first facet to the second facet; and an output side, opposite the input side, configured to display the image, wherein the third facet scatters light traveling from the output side to the third facet.
 2. The Fresnel lens of claim 1, wherein the third facet scatters input light reflected back to the third facet from the output side.
 3. The Fresnel lens of claim 1, wherein the third facet scatters ambient light traveling to the third facet through the output side.
 4. The Fresnel lens of claim 1, wherein the third facet is substantially flat.
 5. The Fresnel lens of claim 1, wherein the output side and the third facet are substantially parallel.
 6. The Fresnel lens of claim 1, wherein the third facet is blackened.
 7. The Fresnel lens of claim 1, wherein the third facet includes variable surface topography.
 8. The Fresnel lens of claim 1, wherein the third facet has a depth relative to the first facet and the third facet depth is configured based on an angle where an image ray directed through the first facet generates an image at a desired pixel.
 9. The Fresnel lens of claim 1, wherein the third facet has a depth relative to the first facet and the valley floor depth varies relative to a radial distance from a center of the Fresnel lens.
 10. The Fresnel lens of claim 1, wherein the first facet, the second facet and the third facet collectively constitute a light input unit and where the light input unit is one of a plurality of substantially similar light input units.
 11. The Fresnel lens of claim 10, wherein the third facet has a different depth relative the first facet for different light input units.
 12. A display device comprising: a lens system to project an image; an intermediate mirror to reflect the image projected by the lens system; a back plate mirror to receive the image from the intermediate mirror and to reflect the image; and a Fresnel lens having an input side configured to receive the image from the back plate mirror and an output side, opposite the input side, configured to display the image, wherein the input side includes a first facet angled to receive input light constituting at least a portion of the image, a second facet at least partially facing the first facet, and a third facet linking the first facet to the second facet, wherein the third facet and the output side are substantially parallel.
 13. The display device of claim 12, wherein the intermediate mirror is substantially planar.
 14. The display device of claim 12, wherein the back plate mirror is substantially planar.
 15. The display device of claim 12, wherein the intermediate mirror, the back plate mirror, and the output side are substantially parallel.
 16. The display device of claim 12, wherein the third facet scatters input light reflected back to the third facet from the output side.
 17. The display device of claim 12, wherein the third facet scatters ambient light traveling to the third facet through the output side.
 18. A Fresnel lens for a display device, the Fresnel lens comprising: an input side configured to receive an image from an image projecting assembly, the input side including: a first facet angled to receive input light constituting at least a portion of the image, a second facet at least partially facing the first facet, and a third facet linking the first facet to the second facet; and an output side, opposite the input side, configured to display the image, wherein the output side and the valley floor are substantially parallel.
 19. The Fresnel lens of claim 18, wherein the third facet scatters input light reflected back to the third facet from the output side.
 20. The Fresnel lens of claim 18, wherein the third facet scatters ambient light traveling to the third facet through the output side. 